There have been proposed various kinds of photosensors employing phase modulators that apply mechanical stress to optical fibers and thereby cause optical phase modulation. Among the known photosensors, there are, for example, the one mentioned in FIGS. 1(a) and 1(b) of Japanese Unexamined Patent Application Publication No. 2006-208080 “Optical fiber vibration sensor” (Patent Document 1), the one mentioned in FIG. 1 of Japanese Unexamined Patent Application Publication No. H06-265361 “Phase modulator and optical rotation detecting apparatus using the same” (Patent Document 2), the one mentioned in FIG. 2 of Published Japanese Translation of PCT Application No. 2002-510795 “Optical fiber acoustic sensor array based on Sagnac interferometer” (Patent Document 3), and the one mentioned in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2007-40884 “Reflection optical fiber current sensor” (Patent Document 4). Phase modulators used with these photosensors include the one mentioned in FIG. 1 of Japanese Utility Model Publication No. H02-6425 “Optical-fiber-type phase modulator” (Patent Document 5) and the one mentioned in FIGS. 1 to 6 and 8 to 10 of Japanese Unexamined Patent Application Publication No. H05-297292 “Optical fiber phase modulator” (Patent Document 6).
The related arts, however, have technical problems mentioned next. A photosensor with the phase modulator mentioned in FIG. 1 of the Patent Document 5 winds a polarization preserving fiber around a piezoelectric element, to form the phase modulator. In the phase modulator of the photosensor, the piezoelectric element expands and contracts to expand and contract the polarization preserving fiber wound around the same in a longitudinal direction. At this time, a propagation constant difference between two light propagation axes of the polarization preserving fiber changes to cause phase modulation to light. If the polarization preserving fiber is randomly wound around the piezoelectric element, a difference in propagated light quantity between the two light propagation axes enlarges. Depending on a way of winding, it will be possible that the phase modulator propagates light through only one light propagation axis. This is because, like an optical fiber polarizer mentioned in FIG. 6 of Japanese Patent Publication No. 3006205 “Optical fiber polarizer” (Patent Document 7) and optical fiber polarizers mentioned in Non-Patent Documents 1 to 5, one of the light propagation axes of the polarization preserving fiber is aligned in a diametrical direction and is wound around a cylindrical winding frame, to form the fiber polarizer. As a result, light in one of the light propagation axes is extinguished. Accordingly, light that passes through the phase modulator formed by winding a polarization preserving fiber around a cylindrical or columnar piezoelectric element without controlling two light propagation axes of the fiber loses effective optical signal quantity components and causes intensity modulation in addition to the phase modulation, thereby deteriorating an original phase modulation function. This causes problems of deteriorating the productivity of phase modulators and lowering the measuring accuracy of the photosensor.
A photosensor with the phase modulator mentioned in FIGS. 1 to 6 and 8 to 10 of the Patent Document 6 is an advanced form of the photosensor mentioned in FIG. 1 of the Patent Document 5. This phase modulator externally applies mechanical stress to an optical fiber, thereby causing phase modulation. The photosensor, like the photosensor of the Patent Document 5, makes no consideration on controlling the two light propagation axes of a polarization preserving fiber. When propagating optical signal components through the two light propagation axes and applying a relative phase modulation to one of them, a loss occurs in a propagated light quantity depending on the directions of the light propagation axes of the polarization preserving fiber. In addition, a lateral pressure applied to the polarization preserving fiber causes a loss in a light quantity, to cause light intensity modulation. This results in deteriorating the measuring accuracy of the photosensor.
The photosensor includes, in addition to the above-mentioned phase modulator, many parts that are formed by winding polarization preserving fibers into coils. An example thereof is the optical fiber polarizer mentioned in FIG. 6 of the Patent Document 7. Other examples include a delay fiber coil in the reflection optical fiber current sensor mentioned in FIG. 1 of the Patent Document 4, a vibration sensor coil part 12 in the optical fiber vibration sensor mentioned in FIG. 1 of the Patent Document 1, and a sensing loop 6 in the phase modulator and optical rotation detecting apparatus using the same mentioned in FIG. 1 of the Patent Document 2. When receiving external vibration or thermal shock, the polarization preserving fiber coil causes resonant vibration and resonant contraction depending on the shape of the coil and the shape of a winding frame. A vibration source will be created when the above-mentioned expansion/contraction vibration of the phase modulator is propagated to a part that is not originally intended. If the resonant vibration or the resonant contraction occurs, the resonance phenomenon causes the polarization preserving fiber to expand and contract, to create a phase difference in the polarization preserving fiber due to the same principle as that of the phase modulator mentioned above. This is an error phase difference that is different from an originally intended controlled phase difference, and therefore, causes a problem of deteriorating the characteristics and measuring accuracy of the photosensor.
The photosensor with a phase modulator may be used with a signal processing unit that calculates a measuring physical quantity according to an optical signal. In this case, the signal processing unit and photosensor are optically connected to each other through a light-transmitting polarization preserving fiber that is in a non-coil shape and is used to transmit an optical signal. If the resonance phenomenon is externally applied to the light-transmitting fiber, an error phase difference occurs on the light propagated through the fiber, to cause the problem of deteriorating the characteristics and measuring accuracy of the photosensor. For example, in the case of a photosensor formed by inserting a light-transmitting polarization preserving fiber into an iron pipe serving as a protective pipe, slight vibration or sound applied to the iron pipe produces a sound or vibration standing wave inside the iron pipe, to cause the resonance phenomenon. The resonance phenomenon causes the polarization preserving fiber to expand and contract, thereby creating an error phase difference in the light propagated through the fiber and deteriorate the characteristics and measuring accuracy of the photosensor. In the case of the photosensor mentioned in FIG. 2 of the Patent Document 3, producing, by an influence of sound, a phase difference in a light-transmitting polarization preserving fiber other than a sensor coil is itself an error, and therefore, reducing the influence of sound on the light-transmitting polarization preserving fiber is a problem that must be solved.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-208080
Patent Document 2: Japanese Unexamined Patent Application Publication No. H06-265361
Patent Document 3: Published Japanese Translation of PCT Application No. 2002-510795
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2007-40884
Patent Document 5: Japanese Utility Model Publication No. H02-6425
Patent Document 6: Japanese Unexamined Patent Application Publication No. H05-297292
Patent Document 7: Japanese Patent Publication No. 3006205
Non-Patent Document 1: F. Deformel, M. P. Varhham, and D. N. Payne: “Finite cladding effects in highly birefringent fibre taper-polarizers”, Electron. Lett., 20, 10, p 398-p 399 (May 1984)
Non-Patent Document 2: M. P. Varnham, D. N. Payne, A.
J. Barlow, and E. J. Tarbox: “Coiled-birefringent-fiber polarizers”, Opt. Lett., 9, 7, p 306-p 308 (July 1984)
Non-Patent Document 3: K. Okamoto: “Single-polarization operation in highly birefringent optical fibers”, Appl. Opt., 23, 15, p 2638-p 2642 (August 1984)
Non-Patent Document 4: K. Okamoto, T. Hosaka, and J. Noda: “High-birefringence polarizing fiber with flat cladding”, IEEE J. Lightwave Technol., LT-3, 4, p 758-p 762 (August 1985)
Non-Patent Document 5: M. J. Messerly, J. R. Onstott, and R. C. Mikkelson: “A broad-band single polarization optical fiber”, IEEE J. Lightwave Technol., 9, 7, p 817-p 820 (July 1991)
Non-Patent Document 6: Muskhelishvili, N. I., “Some basic problems of the mathematical theory of elasticity” (P. Noordhoff, Groningen, Holland, 1953), p 324-328
Non-Patent Document 7: Smith, A. M., Electronics Letters, vol. 16, No. 20, p 773-774, 1980